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Variations of in situ stresses as indicators of active fractures and faults

by M. Lee Allison

Analyses of borehole breakouts show a common characteristic of variations of in situ stresses from regional trends. These varying data are routinely ignored or dismissed as noise or errors. Instead, these stress variations are local stresses that can identify orientations of active fractures within specific geologic layers and the presence of active faults at remote distances from the borehole. Variations of breakouts can thus be used to predict optimum directions for horizontal drilling that may be different from those expected based on regional trends.

The underlying assumption in most studies of borehole breakouts is that regional patterns of stress orientation are most important and variations to them must be thrown out as bad data or at best, averaged into some perceived smooth trend. This destroys valid data and can distort the remaining data.

Breakouts can demonstrate sharp changes in stress orientations between wells in a field and within individual wells. Blocks of rock separated by faults maintain separate stress systems, independent of those across the faults. The localized stress orientations identify the orientation of open fractures or the direction that frac’ing will create fractures. Stress fields associated with active faults can be recognized at distances from the faults and used to predict the orientation and locations of faults that are otherwise unknown. Multiple stress fields may be present in a single geologic interval and may indicate multiply oriented fracture systems and faults in the vicinity.

Examples will be shown from a number of western states and basins.

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